Compositions and methods for planarizing or polishing the surface of a substrate, especially for chemical-mechanical polishing (CMP), are well known in the art. Polishing compositions (also known as polishing slurries) used in CMP processes typically contain an abrasive material in an aqueous solution, and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. The polishing composition is generally used in conjunction with a polishing pad (e.g., polishing cloth or disk). The polishing pad may contain abrasive material in addition to, or instead of, the abrasive material in the polishing composition.
In the semiconductor industry, electric circuits are often fabricated by a process of forming circuit lines on a substrate, typically a dielectric material such as silicon, via the formation of trenches on the substrate surface, followed by application of a barrier layer and then filling of the lined trenches with a metal. Because of the necessity for complete filling of the trenches, the substrate surface is typically coated with a layer of the metal. CMP is then used to polish away the excess of metal down to the dielectric surface or to the barrier layer, thereby leaving metal lines imbedded in the substrate surface.
When applied to metal layers, the CMP process is a combination of abrasion and surface chemistry performed by the chemical components of the polishing composition. Typically, the polishing composition contains components that react chemically with the surface of the metal, converting the metal into a softer, more easily abradable material which is then mechanically removed by the action of abrasive particles suspended in the polishing composition. Since the metal must be completely removed from the surface of the substrate, to prevent interconnections and electrical shorts between adjacent lines, and since generally the metal and underlying barrier layer or dielectric will polish at different rates, the polishing composition must be formulated to remove the metal at an acceptable rate with low defectivity but cannot aggressively overoxidize, or etch, the metal remaining in the trenches, once the polishing process has reached the underlying layer. The over-removal of metal in trenches is referred to as dishing in the art. Thus, there exists a need for methods of optimizing polishing composition chemistry for each given application.
Another challenge in the CMP art is to determine the endpoint of a polishing operation. After removal of a particular material from the substrate surface is complete, it is desirable to terminate the polishing operation, since over-polishing can lead to the aforementioned dishing as well as damage to delicate metal lines on the substrate surface. Because of wafer-to-wafer variability in the depth of metal coating, and because of chemical instability of polishing compositions over time, particularly in polishing compositions comprising oxidizing agents, in a production setting wherein a number of substrates are sequentially polished, the optimal length of time each individual substrate should be polished will vary. Typically, polishing parameters such as polishing time are determined by periodically removing a piece being polished from the polishing process and performing tests to determine the extent and quality of the polishing process at that point, thus allowing development of a polishing protocol. However, due to the aforementioned variability during the polishing process, an unacceptably high number of defective wafers result from overpolishing.
A number of attempts have been made to determine in-process polishing endpoints and to monitor polishing compositions over time. For example, U.S. Pat. No. 5,964,643 describes a polishing system wherein a reflected laser beam is analyzed to determine surface characteristics of a substrate being polished to follow the polishing process in situ. U.S. Pat. No. 6,769,960 describes a system for manufacturing a semiconductor device comprising a measuring apparatus for monitoring a current passing through the polishing composition to detect variations occurring therein over time. However, a need remains for alternative and improved methods for process monitoring in CMP.